4.1.1. Effect of Carburizing Pressure on Surface Carbon Flux

The surface mass increment, average carbon flux changes, and segmented average carbon flux changes were investigated at a carburizing temperature of 950 ◦C, and carburizing pressures of 100, 200, and 300 Pa to determine the effect of carburizing pressure on the surface carbon flux. Figure 7 shows the surface mass increment, average carbon flux, and segmented average carbon flux of 12Cr2Ni4A steel over time.

**Figure 7.** Effect of carburizing pressure on (**a**) mass increment, (**b**) average carbon flux, and (**c**) segmented average carbon flux for 12Cr2Ni4A steel at 950 ◦C.

During the boost stage of 12Cr2Ni4A steel, the mass linearly increased quickly from 0 to 60 s. From 60 to 120 s, the mass gradually decreased, and approached zero after 120 s. The segmented average carbon flux value increased from 1.04 <sup>×</sup> 10−<sup>5</sup> to 1.36 <sup>×</sup> 10−<sup>5</sup> at 30 s, and from 8.11 <sup>×</sup> 10−<sup>6</sup> to 9.58 <sup>×</sup> 10−<sup>6</sup> at 60 s. The segmented average carbon flux values are 5.98 <sup>×</sup> 10−<sup>6</sup> and 7.7 <sup>×</sup> 10−<sup>6</sup> at 90 s, respectively. From 120 to 150 s, the segmented average carbon flux remained nearly identical at the three pressures mentioned above, exhibiting relatively small flux values.

Figure 8 shows the surface mass increment, average carbon flux, and segmented average carbon flux for 16Cr3NiWMoVNbE steel over time.

**Figure 8.** Effect of carburizing pressure on (**a**) mass increment, (**b**) average carbon flux, and (**c**) segmented average carbon flux for 16Cr3NiWMoVNbE steel at 950 ◦C.

During the boost stage for 16Cr3NiWMoVNbE steel, the mass linearly increased relatively quickly with increasing pressure. After 90 s, the mass increment gradually decreased and approached zero after 120 s. The surface carbon flux value increased from 1.17 <sup>×</sup> 10−<sup>5</sup> to 1.72 <sup>×</sup> 10−<sup>5</sup> at 30 s and from 5.2 <sup>×</sup> <sup>10</sup>−<sup>6</sup> to 3.8 <sup>×</sup> <sup>10</sup>−<sup>6</sup> at 60 s. The effect of pressure on the carbon flux was small after 90 s.

Figure 9 shows the surface mass increment, average carbon flux, and segmented average carbon flux of 12Cr2Ni4A steel over carburizing time. During the boost stage, the mass linearly increased relatively quickly from 0 to 90 s. After 90 s, the mass increment gradually decreased and approached zero after 120 s. The surface carbon flux value increased from 6.5 <sup>×</sup> <sup>10</sup>−<sup>6</sup> to 1.3 <sup>×</sup> <sup>10</sup>−<sup>5</sup> at 30 s and from 5.7 <sup>×</sup> 10−<sup>6</sup> to 4.5 <sup>×</sup> 10−<sup>6</sup> at 60 s. After 90 s, the effect of pressure on the carbon flux was small, and the values of the flux were small.

**Figure 9.** Effect of carburizing pressure on (**a**) mass increment, (**b**) average carbon flux, and (**c**) segmented average carbon flux of 18Cr2Ni4WA steel.

The relationship between carburizing pressure and carbon flux of the above three materials shows that under the same carburizing temperature of 950 ◦C and carburizing pressures of 100, 200, and 300 Pa, the mass increment, average carbon flux, and segmented carbon flux of the materials exhibit the same trend. The mass of the samples increased between 30 and 90 s. From 90 to 150 s, the rate of mass increase gradually reduced, the average carbon flux tended to stabilize, and the segmented average carbon flux approached zero. With increasing pressure, the overall carbon flux tended to increase. Under the carburizing pressures of 200 and 300 Pa, the carbon flux values exhibited small differences. As the pressure increased, the time required to saturate the carbon concentration on the sample surface decreased.

The carbon flux value was microscopically characterized as the flow rate of acetylene in contact with the workpiece surface, which is microscopically reflected as the number of activated carbon atoms and the time of contact between the activated carbon atoms and the workpiece. During the boost stage of carburization, the acetylene reaction produced hydrogen gas. As carburization proceeded, the gas in the furnace gradually transformed from acetylene to a mixture of acetylene and hydrogen. The flow rate of the acetylene on the sample surface was influenced by the speed of the acetylene on the workpiece surface and the relative effective density (density of acetylene in the gas mixture in the furnace), both of which are pressure dependent.

The process of low-pressure vacuum carburizing requires stable carburizing pressure and carburizing temperature. When the carburizing temperature, acetylene inlet gas flow, and furnace volume were fixed to maintain the dynamic balance of gas pressure in the furnace, adjusting the outlet pumping speed of the furnace was necessary. At a lower carburizing pressure, the outlet speed was at its highest, the gas exchange efficiency in the furnace was high, the relative effective density of acetylene increased, and the probability of active gas flowing through the surface of the workpiece increased; however, the effective density of active gas in the furnace was low. Considering the two factors, if the outlet pumping speed is too large, the contact time between the effective carbon atoms and the workpiece will be shortened, causing the decomposed active carbon atoms to be pumped out of the furnace without contact with the workpiece. This would further decrease the flow rate of acetylene on the surface of the sample, resulting in a decrease in the carbon flux. With the increase in carburizing pressure in the furnace, the outlet pumping speed gradually decreased, and the density of the carburizing atmosphere in the furnace increased. The number of active carbon atoms increased, and the carbon flux gradually increased. When the carburizing pressure reached a certain value, the gas flow in the furnace decreased, and although the density of the workpiece surface increased, the hydrogen content gradually increased per unit time, and the density of the effective active gas gradually decreased, additionally decreasing the value of carbon flux.

For the three representative materials discussed herein, at a carburizing temperature of 950 ◦C, the value of carbon flux was the smallest at a carburizing pressure of 100 Pa. When the pressure was low and the gas exchange rate was high, part of the decomposed active carbon atoms may have

been pumped out before contacting the surface. When the carburizing pressure increased to 200 Pa, the relative density plays a dominant role in effective carbon flux, the pressure increased, the pumping rate decreased, the impact of the flow rate was small, and the effect of the gas exchange rate was small. The time for which acetylene molecules remained on the surface of the workpiece and the relative density increased. When the pressure increased to 300 Pa, the value of carbon flux was close to that under 200 Pa, indicating that the flow rate and the relative effective density attained equilibrium. Therefore, in actual carburization, a pressure of 300 Pa will have relatively little effect on the process.
